Barely Functional Theories

Musings on science and game design by James Furness.



Sigmoid Functions in Game Design



A lot of the challenge of good procedural generation is in appropriately setting how much of something should be included as a function of something else. Getting this blending right can be a tricky business, and I’ve been slowly collecting a library of mathematical functions that have interesting properties suited to this task. I thought I’d kick off this blog with a post describing my favourite of these functions, a sigmoid function,

$$ f_{S}(x; \sigma) = \frac{x^2}{\sigma^2 + x^2}$$

Sigmoid functions are simply functions that give an S shape when plotted. There are many such functions known, but \(f_{\mathrm{S}}\) has some particularly appealing properties that allow the developer a great deal of control over its shape, whilst staying simple and efficient to evaluate.

I first came across \(f_{\mathrm{S}}\) in the context of Tikhonov Regularisation where it acts as a filter, removing noise from ill-posed matrix equations. After working with it for a while, I started to notice some of its nice characteristics and began to wonder if it can’t be put to some good use in procedural game generation…
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